It's no big surprise, but official now, I guess: Texting is now teens' No. 1 reason for using cellphones, but using apps is their fastest-growing activity, ReadWriteMobile reports, citing Nielsen research. This year 43% of US 13-to-17-year-olds say texting is their top reason for mobile adoption. "Safety, the number one reason back in 2008, has now fallen to second place with only 35% citing this … [Read more...] about US teens’ mobile use: Nielsen update